Introduce the abstraction for credentials (key credential, oauth credential)

[ArcturusZhang]

Since we might be needing to call APIs on credentials, and there are different types between unbranded libraries and azure libraries, we might need to introduce an abstraction for those credential types.

We could defer this work until the work on azure plugin could not be done until we have the abstraction. Before that, maybe a simple type is enough

[JoshLove-msft]

@live1206, do you have an idea of when you will tackle this? Feel free to add the iteration to the issue.